This invention relates to a method for inhibiting the formation of scale, in particular to a method for inhibiting the formation of scale in waters having a high barium content and low pH and to compositions suitable for use in such method.
A scale inhibitor is a compound or mixture of compounds which inhibits crystal growth and thereby reduces the tendency of scale to form.
Scale inhibitors are used in the oil industry. Water flooding is one of the most successful and extensively used secondary recovery methods. Water is injected under pressure into the reservoir via injection wells and drives the oil through the rock into nearby producing wells. In this type of operation it is most important to ensure that the injection water is free from suspended particles or any chemicals or biological substance which might cause blockage of the pore channels in the reservoir rock. The oil in the reservoir is frequently associated with connate water which contains various cations, sometimes including barium, calcium and strontium. Many oil fields are situated in offshore locations and for them the only source of injection water is the sea. Sea water contains sulphate and bicarbonate ions which, in the absence of treatment, would react with alkaline earth metal ions in the connate water to precipitate scale.
In order to prevent this, scale inhibitors are used.
Typical scale inhibitors include sulphonated ethoxylated hydrocarbons, polyacrylates, amine phosphonates and organic polyphosphonic acids.
The main types of scale occurring in oil production operations are calcium carbonate and barium, calcium and strontium sulphates. Conventional scale inhibitors have proved effective in combating these types of scale. However scale formation encountered in reservoirs which have a high barium content in the formation waters coupled with a low pH is difficult to control with these conventional scale inhibitors.
We have now found that certain polyaminomethylene phosphonates may be used as effective scale inhibitors for waters having a high barium content and low pH.
Thus according to the present invention there is provided a method for inhibiting scale formation in waters having a high barium content and low pH which method comprises adding an aqueous solution of a phosphonate of formula: 
wherein
Z=xe2x80x94CHR1PO3R2 
R=H, CH3, C2H5 or M
R1=H, CH3, CR3, C6H5, SO3H2 
M=alkali metal or ammonium ion
n=2-6, preferably 2-4
m=2-6, preferably 2-4
a=2-10, preferably 2-4
b=2-10, preferably 2-6
x=0-6, preferably 0-3
y=0-6, preferably 0-2
provided that when y=o at least one of n or m is greater than 2, said phosphonate being present in an amount effective to inhibit scale formation, to an aqueous medium containing potential scale forming constituents.
Preferred compounds for use in the method of the present invention are those wherein R and R1 are hydrogen.
A particularly preferred compound for use in the method of the resent invention is wherein R, R1=H, n=3, m=3, b=2, x=0 and y=1 represented by the formula: 
By high barium content we mean a content in the range 500-3000 ppm of the original formation water.
By low pH we mean a pH in the range 4-6.
The polyaminomethylene phosphonate scale inhibitors of the present invention may be employed to inhibit the formation of barium sulphate scale in aqueous fluids present in an underground reservoir.
The scale inhibitors may also be suitable for the inhibition of barium sulphate scale in aqueous fluids produced from an underground reservoir.
The scale inhibitors according to the present invention may be suitably employed in either partially or fully neutralised form.
The neutralised scale inhibitors may be in the form of the alkali metal salt or ammonium salt, preferably in the form of the sodium salt.
The scale inhibitors of the present invention may be used either downhole by injection into an underground reservoir as part of a squeeze technique or they may be continuously injected into production fluids obtained from an underground reservoir, for example on an offshore platform.
By squeeze technique we mean a procedure wherein an aqueous solution of a scale inhibitor according to the present invention is injected into an underground reservoir via a production well in fluid communication therewith. The production well may then be shut down to allow the scale inhibitor to be absorbed or precipitated into the reservoir over a period of time. When the oil production is resumed the inhibitor slowly desorbs and dissolves in the produced fluids at sufficient levels to reduce scale formation.
When the scale inhibitor concentration has reduced below an effective level the squeeze technique may be repeated.
The scale inhibitors according to the present invention may suitably be employed as an aqueous solution in seawater or brine.
The scale inhibitors are suitably employed via continuous injection for example on the platform in concentrations in the range 5-200 mg/l preferably in the range 30-150 mg/l or in aqueous solutions as part of the squeeze technique with concentrations in the range 1-50% by weight preferably in the range 2-15% by weight.
The scale inhibitors of the present invention remain active and are thermally stable.
They may be employed alone or as part of a mixture with other scale inhibitors and/or chelating agents, for example EDTA.
According to another aspect of the present invention there is provided a composition for inhibiting scale formation in waters having a high barium content and low pH comprising an aqueous solution having a phosphonate dissolved therein in a quantity effective to inhibit scale formation, said phosphonate having the formula: 
wherein
z=xe2x80x94CHR1PO3R2 
R=H, CH3, C2H5 or M
R1=H, CH3, CR3, C6H5, SO3H2 
M=alkali metal or ammonium ion
n=2-6, preferably 2-4
m=2-6, preferably 2-4
a=2-10, preferably 2-4
b=2-10, preferably 2-6
x=0-6, preferably 0-3
y=0-6, preferably 0-2
provided that when y=0 at least one of n or m is greater than 2. Preferred compositions are those wherein R and R1 are hydrogen.
A particularly preferred composition is wherein R and R1 are hydrogen, n=3, m=3, b=2, x=0 and y=1. The compositions according to this aspect of the present invention may be suitably employed wherein the phosphonate is either partially or fully neutralised. The phosphonate may suitably be in the form of the alkali metal salt or ammonium salt, preferably in the form of the sodium salt.
Compositions according to this aspect of the present invention may be employed to inhibit scale formation either by injection into an underground reservoir or by injection into production fluids as hereinbefore described.
The phosphonate scale inhibitors according to the present invention may be prepared by conventional methods.
For example, with the preferred compounds wherein R=R1=H, by treatment of the appropriate polyamino compound with formaldehyde and phosphorous acid.
Suitable scale inhibitors include compounds of the following formulae: 
The invention is further illustrated with reference to the following Examples.
The preparation of scale inhibitor compounds suitable for use in the method of the present invention is shown in Examples 1 and 2.